"Spying in Guru land" by William Shaw
"Call No Man Master" by Joyce Collin Smith
"In search of heaven on Earth" by Rachel Storm
"In search of PD Ouspensky: The Genius in the Shadow of Gurdjieff" by Gary Lachman
"Madame Blavatsky's Baboon" by Peter Washington
"Secret Cult" by Peter Hounam and Andrew Hogg

Addendum by HoHumBug

You could add:
"A secret history of consciousness" by Gary Lachman
and
"The occult establishment" by Webb (out of print but can be obtained second hand)

Addendum by Justice! Critical books for better understanding of the cult phenomenon